This invention relates to a circuit board for mounting an electronic component and, more particularly, to a circuit board having a large number of leads for connecting one or more electronic components attached to the board to an external circuit.
FIGS. 1 and 2 illustrate one example of a conventional circuit board 110 for mounting an electronic component such as a semiconductor chip. As shown in the FIGS., a plurality of leads 111 have integrally formed on their inner ends inner connection portions 112 and a substrate 113 is provided on both sides of the inner connection portions 112 so that each lead 111 projects from the substrate 113. Also, the inner connection portions 112 are electrically connected to conductors 114 formed on at least one surface of the substrate 113 by through holes 115 so that the inner ends of the conductors 114 may be used as bonding terminals 117 to which bonding wires B for electrically connecting an electronic component A are bonded.
In recent years, it has become very desirable to increase the number of outer leads (input and output pins) of the circuit board, so that a high speed electronic component having a large number of electrodes can be mounted.
However, in the conventional electronic component mounting circuit board 110 as described above, even though the width of the leads 111 is made narrow and the distance between the leads 111 is made small in order to provide as many outer leads 111 as possible on the substrate 113, each lead 111 needs one through-hole connector 115 for internally connecting the leads 111 and the conductors 114. Because the through-hole connector 115 requires a space larger than that for the outer leads 111, the number of the outer leads 111 is still limited by the through-hole connectors 115. A standard dimensional relationship of the transverse dimension of the through-hole connector 115 and the distance between the leads 111 relative to the width of the lead 111 is 2:1.33:1.
FIG. 3 illustrates another known arrangement of the circuit board 110a for mounting an electronic component A, in which an additional insulating substrate 113a having conductors 114a is attached on the top surface of the first substrate 113 and each conductor 114a on the upper substrate 113a is electrically connected by a longer through-hole conductor 115a to the corresponding outer lead 111 to provide a bonding terminal 117a to which the bonding wire B is bonded. Thus, the area in which the bonding wires B and the bonding terminals 117 and 117a are most crowded is made in a "stadium" structure in which the bonding terminals 117 and 117a are located at the different levels for easy wire bonding as illustrated in FIG. 3. Even in this arrangement, however, one through-hole connector 115 or 115a is required for each of the outer leads 111 and the above-discussed problems remain unsolved. Moreover, since the conductors 114 and 114a as well as the bonding terminals 117 and 117a are located at different levels with an electrically insulating substrate therebetween, the overall thickness and the manufacturing cost of the circuit board is increased, increasing the bulk of the device.
Also, Japanese Patent Laid-Open No. 59-98543 discloses a circuit board for mounting an electronic component in which a large number of leads are provided by bonding thin conductive layers to the lead frame with an insulating layer therebetween. However, since the outer leads have a multilayered structure in which thin conductor layers are bonded to the lead frame with the insulating layer sandwiched therebetween, the insulating layer may crack when it is bent at the time of supplying as parts, whereby the electrical insulation between the lead frame and the conductive layer is degraded. Accordingly, the inner and the outer leads must be wired in the same order (i.e., not in a crossing relationship) so as to prevent the inner and outer leads from contacting each other, thereby limiting freedom of design.